A
man wished to purchase an Ass (a Donkey), and decided to give the
animal a test before buying him. He took the Ass home and put him in
the field with his other Asses.

The new Ass strayed from the others to join the one that was the laziest and the biggest eater of them all.

Seeing
this, the man led him back to his owner. When the owner asked how he
could have tested the Ass in such a short time, the man answered, "I
didn't even need to see how he worked. I knew he would be just like the
one he chose to be his friend."

The Ass and His Purchaser summary: There's no getting around the fact that this fable is about judging. While we like to be judged on our merits, the fact is we're likely to be judged based upon other things. If that's a point you'd like to make to your child, here you go. But...

Be aware it could be taken as a message like, "Don't hang out with the kids who get teased if you don't want to be teased yourself."

Grown ups might want to make the useful distinction that while a person is apt to be judged by the company he (or she) keeps, that doesn't mean you have to choose to be like the company you keep.

Notable:
A much more life and death fable with a similar message is contained in
The Farmer and the Stork, the moral of which is, "Birds of a Feather
Flock Together."